The invention pertains to amplifiers for musical instruments. In particular, the invention pertains to a solid state multi-stage amplifier which has distortion so that it sounds like a tube amplifier when overdriven.
Tube amplifiers are often preferred by musical artists because tubes produce a distorted output sound which is familiar and thought to be most pleasing. Solid state amplifiers are often preferred because they tend to be lighter, and are often less expensive to produce, are more durable and consume less power. It is difficult to make a solid state amplifier produce a distorted sound like a tube amplifier. Also, the supply of tubes available for use in amplifiers has become scarce and more expensive.
A known tube amplifier 10 is shown in FIG. 1. The amplifier is described with exemplary values of the various elements being noted for characterizing the operation of the device. The amplifier 10 illustrated in FIG. 1 is a pre-amplifier comprising four identical tube sections 12, 14, 16 and 18 (e.g., four 12AX7 tube sections), each tube section has a corresponding plate resistor 20 (100K ohm) and a cathode resistor 22 (1.5K ohm). Each cathode resistor 22 is bypassed with a capacitor 24 (2.2 uF). With these plate and cathode resistor values, a typical 12AX7 amplifier tube section will idle at approximately 1 mA of plate current, approximately 1.5 volts at the cathode and about +200 volts at the plate from a +300 volt source. A positive grid swing in excess of 1.5 volts peak will cause the grid to conduct. A normal guitar input is coupled to the grid of the first tube stage 12 by a coupling capacitor 34 and a grid resistor 36. A resistor 38 is coupled to the node between the capacitor 34 and grid resistor 38 and provides a ground reference for the input to tube 12. Feedback capacitor 40 (10 PF) is coupled between the plate and grid of tube 12 and provides some control of high frequency roll-off, known as the Miller effect, which helps to keep the amplifier stable at open input conditions. The signal from the plate of amplifier 12 is coupled to input of amplifier stage 14 via capacitor 42 and grid resistor 44. Resistor 46 provides a ground reference for stage 14. Resistors 44 and 46 act as a voltage divider. The signal from stage 14 is likewise coupled to stage 16 via capacitor 48 and grid resistor 50, with resistor 52 providing a ground reference for the input and voltage division. Finally, the signal from stage 16 is coupled to the stage 18 via capacitor 54, grid resistor 56 and reference resistor 58 to ground also with voltage division. The output of stage 18 is coupled to the output of the pre-amplifier 10 by output capacitor 60.
The coupling capacitor values 42, 48 and 54, as well as the values of the divider resistances 44/46, 50/52 and 56/58 are chosen in a known manner to provide good distorted sound. Typically, with a guitar level input signal applied, the first stage 12 is clean and free of distortion, although with some high level guitars, even this stage clips at times. The first stage output signal level is high enough to cause input clipping at the second stage 14 because the grid of the second stage 14 is driven positive with respect to the cathode and conducts for a substantial portion of the input cycle. Input clipping at stage 14 results in an average negative voltage on the grid, causing the operating point of the second stage 14 to shift dramatically resulting in a significant amount of second harmonic distortion. The signal at the plate of the second stage 14 resembles a square wave with about two-thirds of the period spent in the positive half cycle. The plate of the second stage 14 has a high enough signal level to cause significant input clipping at the third stage 16. Here too, the grid swings positive with respect to the cathode. Thus, input clipping causes the operating point of the third stage 16 to shift. This is repeated yet one more time, resulting in input clipping and operating point shift of the fourth stage 18. The output at the plate of the fourth stage 18 has gone through several different levels of clipping at the input and output and several operating point shifts and is thus rich in harmonics. All of this essentially results in a characteristic sound which is referred to as good tube sound.
In the exemplary pre-amplifier 10 illustrated in FIG. 1, the available peak plate swing in the positive direction for any stage is about 100 volts (i.e., about one-half the plate voltage). Further, each grid conducts at a positive peak swing of about 1.5 volts. The ratio of 100 to 1.5 or 66.7 is a high number, and its value is important to shift the operating point of each stage enough to generate the appropriate amount of second harmonic distortion. The values of divider resistors 44/46, 50/52 and 56/58 are also critical, and are carefully chosen to set just the right amount of input clipping and resulting second harmonic distortion to produce a pleasing sound.
Of note here are two key ingredients in the so called distorted tube sound. First, the tube characteristics themselves with the 100 volt output capability and only a 1.5 volt input clipping capability or level is unique and required for successful generation of the second harmonic distortion, and that so called tube sound. Secondly, the multiplicity of stages is necessary for a sustained distortion sound as the guitar output level drops after being plucked by the musician. Although more or fewer stages may be employed, at least three, and preferably four stages are required to achieve the desired distortion sound sought by most musicians.
There is, therefore, a need for a solid state amplifier which is capable of replacing the various tube stages in a multi-stage pre-amplifier, and which may be overdriven to emulate the tube sound produced by the such known tube amplifiers.